The present invention relates generally to horizontal AFC circuits (horizontal automatic frequency control circuits), and more particularly to a horizontal AFC circuit in a television receiver for receiving a reproduced video signal from an apparatus for magnetically recording and reproducing a video signal.
In general, there is a video signal magnetic recording and reproducing apparatus of a type which records and reproduces video signals on and from slant tracks on a magnetic tape for each field alternately by means of two rotating magnetic heads, for example. In a video signal magnetic recording and reproducing apparatus of this type, the reproduced video signals from the rotating heads are switched with vertical periods and are led out as a continuous series of reproduced video signals.
In this video signal magnetic recording and reproducing apparatus, however, if there is an occurrence such as timing lag of the switching time instants of the output signals of the two heads or stretching or contraction of the magnetic tape, phase lag will occur in the series of signals at the signal switching instant, whereby skew distortion will develop in the reproduced picture of the television receiver. The phase lag in that event develops in the manner of a step function.
When a phase lag of a step function occurs in this manner in a reproduced signal supplied to a television receiver, a transient response develops in the phase characteristic of the television receiver. In order to reduce the effect of the distortion in the reproduced picture which accompanies this phase lag, it is necessary to shorten the time of the above mentioned transient response. This transient response may be shortened by increasing the AC loop gain of the horizontal AFC circuit of the television receiver.
However, when the AC loop gain is made large, the passing quantity of the high-frequency band component of the filter provided in the horizontal AFC circuit increases. For this reason, the degree to which the horizontal AFC circuit system is disturbed by the noise component becomes large. Especially in the case of reception of television broadcasting waves in an area of weak electric field, the disorder in the horizontal direction of the reproduced picture becomes large. Consequently, it is difficult, in principle, to attain simultaneously both shortening of the transient response time and improvement of the noise characteristic.
Accordingly, in a horizontal AFC circuit of the prior art, an AC loop gain changeover switch is provided. By operating this switch, the AC loop gain is caused to be of relatively low value in order to improve the noise characteristic in the case of reception of television broadcast waves since the skew distortion is small in this case. In the case where a signal reproduced from a video signal magnetic recording and reproducing apparatus is reproduced into a picture, the switch is operated to cause the AC loop gain to be a relatively large value in order to shorten the transient response time since the skew distortion is large in this case.
However, in either case, improvement of both the transient response characteristic and the noise suppression characteristic cannot be achieved, and in either mode of picture reproduction, one of these two characteristics is unavoidably sacrificed. In this known horizontal AFC circuit, furthermore, it has been necessary to carry out troublesome manipulation of the above mentioned changeover switch in accordance with the mode of picture reproduction.